Addition products of alkylene oxides onto alcohols, acids or amines are important industrial products which find a variety of uses, especially as nonionic surfactants.
The alkoxylation is typically performed batchwise, for example in stirred autoclaves or loop reactors, at temperatures between 80 and 200° C.; alternatively, the liquid reaction mixture can also be dispersed into an alkylene oxide-containing gas phase. On this subject, reference is made by way of example to a review article in Ind. Eng. Chem. Res 25, 9482-9489 (2005). Typically, a compound with the nucleophilic site—for example an alcohol, a carboxylic acid, an ester or an amine—is initially charged together with the catalyst and then the desired amount of alkylene oxide is injected, which generally establishes a pressure of up to 12 bar depending on the temperature. Suitable catalysts are basic compounds, for example alkali metal alkoxides, or Lewis acids, the latter having the disadvantage that they tend to form considerable amounts of unwanted polyglycol ethers. All of these processes involve a liquid reaction mixture and an alkylene oxide-containing gas phase, which is afflicted in generic terms with the problem of inadequate mass transfer at the phase interface and hence reduced conversion.
The prior art discloses different processes to prepare these alkylene oxide addition products. Representatives mentioned here will be only the following publications, each of which discloses specific design configurations for alkoxylation reactors: EP 0853975 A1 (CP Tech), U.S. Pat. No. 5,811,595 (Ellis), U.S. Pat. No. 6,180,728 (Linde), U.S. Pat. No. 6,342,199 (Ellis), U.S. Pat. No. 6,673,972 (BASF), U.S. Pat. No. 6,838,061 (BASF), US 2005/0107643 A1 (BASF).
Frequently encountered in practice are batchwise or semicontinuous processes which are performed in stirred reactors. A disadvantage is the low mass transfer, since the alkylene oxides react only very slowly in the liquid phase even with high input of stirring energy. As a consequence of the low reaction rate, the alkylene oxide accumulates under particular conditions. This accumulation can lead to safety-critical runaway reactions. Moreover, this connection, by its nature, leads to long reaction times which are undesired for economic reasons; in addition, the color of the products may deteriorate.
The prior art additionally discloses the so-called “Venturi loop reactor”, which consists essentially of a reactor with an internal throttle (“Venturi nozzle”). The starting materials are metered into the Venturi nozzle at the top of the reactor and atomized, and then form a liquid phase in the reactor, which passes through a bottom discharge valve into an external loop and is then conducted in circulation to the top of the reactor. The alkylene oxides are metered in liquid form upstream of the Venturi nozzle. In addition, ethylene oxide vapors are sucked into the Venturi nozzle from the gas phase, such that there is particularly intense mixing here [cf. EP 0070797 A1, EP 0419419 B1, U.S. Pat. No. 5,159,092, Buss]. A disadvantage is, however, that the dimensions of the reactor are fixed in this way by the size of the Venturi nozzle and there is a particular minimum size owing to the hydrodynamic conditions. In addition, the conversions are better compared to the stirred reactor, but still by no means truly satisfactory. In addition, this reactor type requires an external circulation system, which necessitates a minimum amount of raw material. The preparation of small amounts of product or of products with high growth rates, as is typical of specialty chemicals, cannot be achieved in this way. This becomes clear when the so-called growth rate in the preparation of the alkylene oxide addition products is considered, which is defined as follows:
      Growth    ⁢                  ⁢    rate    ⁢                  ⁢    W    =            Volume      ⁢                          ⁢      of      ⁢                          ⁢      the      ⁢                          ⁢      reaction      ⁢                          ⁢              products        ⁢                                  [                  m          3                ]                    Starting      ⁢                          ⁢      volume      ⁢                          ⁢      of      ⁢                          ⁢      starting      ⁢                          ⁢              materials        ⁢                                  [                  m          3                ]            
In the case of use of the Venturi loop reactor, a growth rate between 2 and 12 is possible, but a truly flexible reactor which enables the preparation even of small amounts of product would have to have a growth rate of at least 25, better 40 to 50.
It was consequently an object of the present invention to provide a process and an apparatus for preparing alkylene oxide addition products, which firstly has a higher flexibility compared to the prior art and enables growth rates greater than 40, preferably greater than 50, and secondly achieves higher conversions within shorter reaction times, without the product quality, especially the color, suffering therefrom.